Concentration Gradient Generator Research Articles

Electrochemical Generation of Steady-State Linear Concentration Gradients within Microfluidic Channels Perpendicular to the Flow Field.

A new design of microelectrode was introduced to generate electrochemically steady-state linear concentration gradients perpendicular to the flow direction throughout the cross section of microchannels. The shape and geometry of the electrode were established based on operating regimes at microchannel electrodes. Before implementation, optimal conditions were preliminary delineated by numerical simulations according to the flow velocity and microchannel dimensions. To assess experimentally these predictions, a specific microfluidic platform was developed with optimized geometry to simultaneously allow the generation of linear concentration gradients and the mapping of concentration profiles by confocal fluorescence microscopy. As a model, the electrochemical reduction of a quinone in the presence of fluorescein was selected to both generate and monitor a linear proton gradient. A good agreement was observed between theoretical and experimental data, establishing the proof of concept. These results should broaden the performance and applications of electrochemical platforms, particularly in the field of active control of microenvironments in biology, biochemistry, and analytical chemistry.

Efficient Drug Screening and Nephrotoxicity Assessment on Co-culture Microfluidic Kidney Chip

The function and susceptibility of various drugs are tested with renal proximal tubular epithelial cells; yet, replicating the morphology and kidneys function using the currently available in vitro models remains difficult. To overcome this difficulty, in the study presented in this paper, a device and a three-layer microfluidic chip were developed, which provides a simulated environment for kidney organs. This device includes two parts: (1) microfluidic drug concentration gradient generator and (2) a flow-temperature controlled platform for culturing of kidney cells. In chip study, renal proximal tubular epithelial cells (RPTECs) and peritubular capillary endothelial cells (PCECs) were screened with the drugs to assess the drug-induced nephrotoxicity. Unlike cells cultured in petri dishes, cells cultured in the microfluidic device exhibited higher performance in terms of both cell growth and drug nephrotoxicity evaluation. It is worth mentioning that a significant decrease in cisplatin-induced nephrotoxicity was found because of the intervention of cimetidine in the microfluidic device. In conclusion, the different in the cell performance between the microfluidic device and the petri dishes demonstrates the physiological relevance of the nephrotoxicity screening technology along with the microfluidic device developed in this study. Furthermore, this technology can also facilitate the development of reliable kidney drugs and serve as a useful and efficient test-bed for further investigation of the drug nephrotoxicity evaluation.

Chemotactic Responses of Jurkat Cells in Microfluidic Flow-Free Gradient Chambers.

Gradients of soluble molecules coordinate cellular communication in a diverse range of multicellular systems. Chemokine-driven chemotaxis is a key orchestrator of cell movement during organ development, immune response and cancer progression. Chemotaxis assays capable of examining cell responses to different chemokines in the context of various extracellular matrices will be crucial to characterize directed cell motion in conditions which mimic whole tissue conditions. Here, a microfluidic device which can generate different chemokine patterns in flow-free gradient chambers while controlling surface extracellular matrix (ECM) to study chemotaxis either at the population level or at the single cell level with high resolution imaging is presented. The device is produced by combining additive manufacturing (AM) and soft lithography. Generation of concentration gradients in the device were simulated and experimentally validated. Then, stable gradients were applied to modulate chemotaxis and chemokinetic response of Jurkat cells as a model for T lymphocyte motility. Live imaging of the gradient chambers allowed to track and quantify Jurkat cell migration patterns. Using this system, it has been found that the strength of the chemotactic response of Jurkat cells to CXCL12 gradient was reduced by increasing surface fibronectin in a dose-dependent manner. The chemotaxis of the Jurkat cells was also found to be governed not only by the CXCL12 gradient but also by the average CXCL12 concentration. Distinct migratory behaviors in response to chemokine gradients in different contexts may be physiologically relevant for shaping the host immune response and may serve to optimize the targeting and accumulation of immune cells to the inflammation site. Our approach demonstrates the feasibility of using a flow-free gradient chamber for evaluating cross-regulation of cell motility by multiple factors in different biologic processes.

Systematic Investigation of Insulin Fibrillation on a Chip.

A microfluidic protein aggregation device (microPAD) that allows the user to perform a series of protein incubations with various concentrations of two reagents is demonstrated. The microfluidic device consists of 64 incubation chambers to perform individual incubations of the protein at 64 specific conditions. Parallel processes of metering reagents, stepwise concentration gradient generation, and mixing are achieved simultaneously by pneumatic valves. Fibrillation of bovine insulin was selected to test the device. The effect of insulin and sodium chloride (NaCl) concentration on the formation of fibrillar structures was studied by observing the growth rate of partially folded protein, using the fluorescent marker Thioflavin-T. Moreover, dual gradients of different NaCl and hydrochloric acid (HCl) concentrations were formed, to investigate their interactive roles in the formation of insulin fibrils and spherulites. The chip-system provides a bird’s eye view on protein aggregation, including an overview of the factors that affect the process and their interactions. This microfluidic platform is potentially useful for rapid analysis of the fibrillation of proteins associated with many misfolding-based diseases, such as quantitative and qualitative studies on amyloid growth.

On-chip MIC by Combining Concentration Gradient Generator and Flanged Chamber Arrays.

Minimum inhibition concentration (MIC) of antibiotic is an effective value to ascertain the agent and minimum dosage of inhibiting bacterial growth. However, current techniques to determine MIC are labor intensive and time-consuming, and require skilled operator and high initial concentration of bacteria. To simplify the operation and reduce the time of inhibition test, we developed a microfluidic system, containing a concentration generator and sub-micro-liter chambers, for rapid bacterial growth and inhibition test. To improve the mixing effect, a micropillar array in honeycomb-structure channels is designed, so the steady concentration gradient of amoxicillin can be generated. The flanged chambers are used to culture bacteria under the condition of continuous flow and the medium of chambers is refreshed constantly, which could supply the sufficient nutrient for bacteria growth and take away the metabolite. Based on the microfluidic platform, the bacterial growth with antibiotic inhibition on chip can be quantitatively measured and MIC can be obtained within six hours using low initial concentration of bacteria. Overall, this microfluidic platform has the potential to provide rapidness and effectiveness to screen bacteria and determine MIC of corresponding antibiotics in clinical therapies.

Abnormal Liquid Chasing Effect in Paper Capillary Enables Versatile Gradient Generation on Microfluidic Paper Analytical Devices.

Microfluidic paper analytical devices (μPADs) are smart and accessible substituents to traditional counterparts in point-of-care tests (POCT), which exploited delicate control over passive fluid in microscale for rich functions. In this work, we are extending such control by introducing novel ways to generate 1D and 2D gradients on μPADs. It is achieved by using paper-capillary-based serial sampling. The paper capillary is composed of a concaved paper channel sealed with tape, with test pads properly distributed aside. In such a structure, the liquid can not only quickly and automatically flow along the capillary but also be continuously consumed by the peripheral test pads. Thus, when we do serial sampling, an abnormal liquid chasing effect can be observed, where the later liquid sample chases and surpasses the earlier parts in the paper capillary, leading to reversely ordered sample distribution compared with that in a typical glass capillary. This specialty allows for fast, ordered, and tunable sequential sampling and enables efficient generation of 1D and 2D concentration gradient with one, two, and even three components on μPADs. Besides, we verified the applicability of this technique for arrayed assays, including 1D serial dilution-based metal ion colorimetry and a 2D bacterial antibiotic susceptibility test for synergic effect evaluations, which paves the way for high-throughput sample analysis and information-rich condition screening on μPADs.

Biodetection using a ZnO nanorod-based microfluidic device with a concentration gradient generator

An effective microfluidic device with a concentration gradient generator for cancer biomarker detection is developed by integrating ZnO NR arrays into microchannels.

Surrogate-based optimization with adaptive sampling for microfluidic concentration gradient generator design.

This paper presents a surrogate-based optimization (SBO) method with adaptive sampling for designing microfluidic concentration gradient generators (μCGGs) to meet prescribed concentration gradients (CGs). An efficient physics-based component model (PBCM) is used to generate data for Kriging-based surrogate model construction. In a comparative analysis, various combinations of regression and correlation models in Kriging, and different adaptive sampling (infill) techniques are inspected to enhance model accuracy and optimization efficiency. The results show that the first-order polynomial regression and the Gaussian correlation models together form the most accurate model, and the lower bound (LB) infill strategy in general allows the most efficient global optimum search. The CGs generated by optimum designs match very well with prescribed CGs, and the discrepancy is less than 12% even with an inherent limitation of the μCGG. It is also found that SBO with adaptive sampling enables much more efficient and accurate design than random sampling-based surrogate modeling and optimization, and is more robust than the gradient-based optimization for searching the global optimum.

Erythrocyte plasma membrane potential: past and current methods for its measurement.

The plasma membrane functions both as a natural insulator and a diffusion barrier to the movement of ions. A wide variety of proteins transport and pump ions to generate concentration gradients that result in voltage differences, while ion channels allow ions to move across the membrane down those gradients. Plasma membrane potential is the difference in voltage between the inside and the outside of a biological cell, and it ranges from ~- 3 to ~- 90 mV. Most of the most significant discoveries in this field have been made in excitable cells, such as nerve and muscle cells. Nevertheless, special attention has been paid to some events controlled by changes in membrane potential in non-excitable cells. The origins of several blood disorders, for instance, are related to disturbances at the level of plasma membrane in erythrocytes, the structurally simplest red blood cells. The high simplicity of erythrocytes, in particular, made them perfect candidates for the electrophysiological studies that laid the foundations for understanding the generation, maintenance, and roles of membrane potential. This article summarizes the methodologies that have been used during the past decades to determine Δψ in red blood cells, from seminal microelectrodes, through the use of nuclear magnetic resonance or lipophilic radioactive ions to quantify intra and extracellular ions, to continuously renewed fluorescent potentiometric dyes. We have attempted to highlight the advantages and disadvantages of each methodology, as well as to provide a description of the technical aspects involved.

A Microfluidic Prototype System towards Microalgae Cell Separation, Treatment and Viability Characterization.

There are a huge number, and abundant types, of microalgae in the ocean; and most of them have various values in many fields, such as food, medicine, energy, feed, etc. Therefore, how to identify and separation of microalgae cells quickly and effectively is a prerequisite for the microalgae research and utilization. Herein, we propose a microfluidic system that comprised microalgae cell separation, treatment and viability characterization. Specifically, the microfluidic separation function is based on the principle of deterministic lateral displacement (DLD), which can separate various microalgae species rapidly by their different sizes. Moreover, a concentration gradient generator is designed in this system to automatically produce gradient concentrations of chemical reagents to optimize the chemical treatment of samples. Finally, a single photon counter was used to evaluate the viability of treated microalgae based on laser-induced fluorescence from the intracellular chlorophyll of microalgae. To the best of our knowledge, this is the first laboratory prototype system combining DLD separation, concentration gradient generator and chlorophyll fluorescence detection technology for fast analysis and treatment of microalgae using marine samples. This study may inspire other novel applications of micro-analytical devices for utilization of microalgae resources, marine ecological environment protection and ship ballast water management.

A tool for designing tree-like concentration gradient generators for lab-on-a-chip applications

Concentration gradient generators (CGGs) help biologists to perform large scale, fast and high-throughput experiments. This paper introduces a design tool called Tree-like Concentration gradient generator Design Tool (TCDT). The performance of this tool is validated both numerically and experimentally. Three CGGs were fabricated using three different fabrication methods and design parameters. The performance of these devices was examined using the measurement of fluorescent and dye intensity. The performance of the design tool for non-linear and multi-drug concentration gradient generations was investigated as well. In addition, a method was developed to investigate the multi-drug concentration effect, which can be the starting point for off-the-shelf usage of the CGGs in personalized medicine. Experimental data, as well as numerical results, confirmed that the output concentrations of the CGGs were within five percent of the values predicted by the design tool. This open-source software is available online for public use.

Electrochemical Generation and Detection of Transient Concentration Gradients in Microfluidic Channels. Theoretical and Experimental Investigations.

Transient concentration gradients generated and detected electrochemically in continuous flow microchannels were investigated by numerical simulations and amperometric measurements. Operating conditions including device geometry and hydrodynamic regime were theoretically delineated for producing gradients of various profiles with tunable characteristics. Experiments were carried out with microfluidic devices incorporating a dual-channel-electrode configuration. Under these conditions, high electrochemical performance was achieved both to generate concentration gradients and to monitor their dynamics along linear microchannels. Good agreement was observed between simulated and experimental data validating predictions between gradient properties and generation conditions. These results demonstrated the capability of electrochemical microdevices to produce in situ tunable concentration gradients with real-time monitoring. This approach is versatile for the active control in microfluidics of microenvironments or chemical gradients with high spatiotemporal resolution.

Rapid Prototyping of an Open-Surface Microfluidic Platform Using Wettability-Patterned Surfaces Prepared by an Atmospheric-Pressure Plasma Jet.

Open-surface microfluidics is promising in terms of enabling economical and rapid biochemical analysis for addressing challenges associated with medical diagnosis and food safety. To this end, we present a simple and economical approach to develop an open-surface microfluidic platform suitable for facile liquid transport and mixing. Customizable patterns with tailored wettability are deposited using a plasma-assisted deposition technique under atmospheric pressure. The flow of the dispensed liquid is driven by gravity, and the tilting angle of the device determines the extent of mixing. First, a hexamethyldisiloxane film was deposited to create hydrophobic patterns on glass, and then, hydrophilic acrylic acid was deposited by a patterned cardboard mask to construct a channel suitable for forming channels to transport aqueous liquids without the need of an external energy input; the liquid can be confined to designated pathways. Several designs including Y-junctions, serpentine-shaped patterns, splitting channels, and concentration gradient generation patterns are presented. The proposed method can spatially pattern a surface with a hydrophobic/hydrophilic area, which can function as a microfluidic channel, and the surface can be applied in microfluidic devices with other types of substrates.

Flowrate independent 3D printed microfluidic concentration gradient generator

Christmas tree like microfluidic concentration gradient generators (MCGG) and other geometries as well, rely on molecular diffusion to achieve concentration gradients. This condition limits the flowrate in the case of continuous flow gradient generators in order for complete mixing to be achieved in a branch before reaching the next bifurcation. Therefore these MCGG work well at low Peclet numbers (Pe). In this work, a different concept of MCGG is presented where accurate and arbitrary dosing can be achieved prior to mixing, making this simple microfluidic design flowrate independent. This means that arbitrary concentration profiles can be achieved, at any flowrate in the laminar flow regime. In order to show the performance of this MCGG, both experiments and numerical simulations were performed. The results were compared to a classical Christmas tree MCGG. The experimental microfluidic chips were 3D printed with a commercial low cost LCD resin printer and commercial grade water cleaning clear resin.

Automatic Generation of Drug Concentration Gradient by a Modified Ink-jet Printer and its Application in Drug Screening

Automatic Generation of Drug Concentration Gradient by a Modified Ink-jet Printer and its Application in Drug Screening

Development of Operando Confocal Microprobe X-ray Fluorescence Techniques to Measure Cation Transport in PEM Fuel Cells

State-of-the-art polymer electrolyte membrane (PEM) fuel cells often use PtM (M= Co or Ni) as cathode catalysts and Ce-based additives in the membrane to enhance their performance and durability. During operation, however, the transition metals dissolve and Co2+ and Ce3+ cations transport from their initial positions, which decreases their efficacy and can cause fouling of ionomer regions, resulting in performance losses and local membrane failure.1 Despite general knowledge of cation transport through diffusion, migration, and convection, a thorough accounting of the cation positions and concentrations has not been available owing to an inability to monitor these processes under realistic MEA operating conditions and cell geometries. Our approach has been to leverage the unique capabilities of synchrotron microprobe X-ray fluorescence (µXRF), which enable point data collection on the millisecond-scale with spot sizes of <0.25 µm. Here, we report the development of techniques to directly observe transient, µm-scale migration of cations, in operando. An initial cell design to isolate 1-D through-thickness cation transport was tested using traditional µXRF at the Advanced Photon Source (APS). These experiments resulted in unexpected depletion of cations from the analysis surface of the cell, more preferentially from cathode side. Results agreed qualitatively with 3-D cell modeling, which suggested that steep potential gradients form between the air/cell interface and the interior of the cell, which drives cation migration deep into the cell.2 In order to map ions beyond the front cell edge, follow-on experiments utilized the confocal µXRF capabilities at the Cornell High Energy Synchrotron Source (CHESS). This setup relies on Si collimating optics to define a fluorescent voxel which can be controlled to generate 3-D elemental maps, enabling the measurement of cation transport in the interior of the cell. Steady state depth scans taken at 50 mA/cm2 show the depletion of ions from the surface (Figure 1a, inset), more preferentially from the cathode side, in agreement with the APS experiments. As shown in Figure 1, these depth profile scans were used to determine an optimized depth of ~180 µm, where depth profiles converged and were located far enough away to avoid edge effects, but near enough to the surface to get sufficient signal (~90% attenuation measured at a depth of 200 µm). Transient scans at this optimized depth were performed at 80°C and 50% RH with a current density of 100 mA/cm2. These measurements, shown in Figure 1b, reveal that Ce3+ cations initially situated in the PEM migrate from there into cathode catalyst layer (cCL), forming a gradient in the PEM after 15 minutes. Upon removal of load, Ce3+ rapidly diffuses and equilibrates between ionomer regions of the CL and PEM. These results demonstrate that cation transport in the MEA has a rapid response to the operating condition and generates concentration gradients under load, which validates the model findings and improves the understanding of the root cause of the performance loss. For the first time in fuel cell study, 3-D cation transport inside the MEA was monitored during cell operation. The findings and techniques developed in this study have significant implications for future designs of cell geometry and operating conditions. These results demonstrate that cation transport in the MEA has a rapid response to the operating condition and generates concentration gradients under load, which validates the model findings and improves the understanding of the root cause of the performance loss. For the first time in fuel cell study, 3-D cation transport inside the MEA was monitored during cell operation. The findings and techniques developed in this study have significant implications for future designs of cell geometry and operating conditions. Acknowledgements This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Fuel Cells Program Manager: Dimitrios Papageorgopoulos and Technical Development Manager: Greg Kleen). This work is based upon research conducted at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357 and the Cornell High Energy Synchrotron Source (CHESS) which is supported by the National Science Foundation under award DMR-1332208. References A. M. Baker, R. Mukundan, D. Spernjak, E. J. Judge, S. G. Advani, A. K. Prasad, and R. L. Borup, J. Electrochem. Soc., 163, F1023–F1031 (2016).Y. Cai, J. M. Ziegelbauer, A. M. Baker, W. Gu, R. S. Kukreja, A. Kongkanand, M. F. Mathias, R. Mukundan, and R. L. Borup, J. Electrochem. Soc., 165, F3132–F3138 (2018). Figure 1

Rapid Softlithography Using 3D‐Printed Molds

AbstractPolydimethylsiloxane (PDMS) is a long‐standing material of significant interest in microfluidics due to its unique features. As such, rapid prototyping of PDMS‐based microchannels is of great interest. The most prevalent and conventional method for fabrication of PDMS‐based microchips relies on softlithography, the main drawback of which is the preparation of a master mold, which is costly and time‐consuming. To prevent the attachment of PDMS to the master mold, silanization is necessary, which can be detrimental for cellular studies. Additionally, using coating the mold with a cell‐compatible surfactant adds extra preprocessing time. Recent advances in 3D printing have shown great promise in expediting microfabrication. Nevertheless, current 3D printing techniques are sub‐optimal for PDMS softlithography. The feasibility of producing master molds suitable for rapid softlithography is demonstrated using a newly developed 3D‐printing resin. Moreover, the utility of this technique is showcased for a number of widely used applications, such as concentration gradient generation, particle separation, cell culture (to show biocompatibility of the process), and fluid mixing. This can open new opportunities for biologists and scientists with minimum knowledge of microfabrication to build functional microfluidic devices for their basic and applied research.

An integrated microfluidic chip for treatment and detection of microalgae cells

As the most economic means of international transportation for massive cargo, marine transportation contributes significantly to the world trading and economics. However, there are large number of microalgae cells and other exotic organisms carried in the ballast water, which may seriously damage the marine ecological environment along the transportation routes of the ships. Microalgae cells and online treatment of the microalgae-rich ballast water have become a pressing demand for ballast water management. Herein, we developed an automatic microfluidic system based on laser-induced chlorophyll fluorescence for real-time detection of microalgae in ballast water samples. This system is highly advantageous compared to the conventional method that requires tedious operation procedures, prolonged detection time, and complex analytical process. Moreover, we introduced chemical methods for fast and effective treatment of the microalgae cells in ballast water. For this purpose, a microfluidic concentration gradient generator was used for fast screening the optimal concentration of chemicals to rapidly reduce the microalgae population in the corresponding ballast water. To the best of our knowledge, this is the first time to combine an automatic high-throughput detection system and a gradient concentration chip for in situ analysis and treatment of ballast water.

Localization of Ion Concentration Gradients for Logic Operation.

Adjustment of the environmental acidity is a powerful method for fine-tuning the outcome of many chemical processes. Numerous strategies have been developed for the modification of pH in bulk as well as locally. Electrochemical and photochemical processes provide a powerful approach for on-demand generation of ion concentration gradients locally at solid-liquid interfaces. Spatially organized in individual way electrodes provide a particular pattern of proton distribution in solution. It opens perspectives to iontronics which is a bioinspired approach to signaling, information processing, and storing by spatial and temporal distribution of ions. We prove here that soft layers allow to control of ion mobility over the surface as well as processes of self-organization are closely related to change in entropy. In this work, we summarize the achievements and discuss perspectives of ion gradients in solution for information processing.

Rapid, one-step preparation of SERS substrate in microfluidic channel for detection of molecules and heavy metal ions.

On-chip fabrication of surface-enhanced Raman spectroscopy (SERS)-active materials enables continuous, real-time sensing of targets in the microfluidic chip. However, the current techniques require the time-consuming, complicated process and costly, bulky facilities. In this work, we present a novel method for synthesis of Ag nanostructures in a microfluidic channel via one-step electroless galvanic replacement reaction. The whole reaction could be achieved <10 mins, while the traditional methods take hours. The microfluidic channel has a Cu base, which can reduce Ag ions to Ag nanoparticles in the presence of AgNO3 solution. The new technique enables the label-free sensing of chemical molecules (i.e., methylene blue) and biomolecules (i.e., urea). Two proof-of-concept experiments are performed to verify the utilization of the prepared SERS substrate. First, the microfluidics-assisted SERS sensor is used to detect Hg ions in aqueous solution with high sensitivity and good selectivity. Second, the fabricated SERS-active material can couple with a concentration gradient generator for continuous SERS detection. This simple technique can be used in any laboratory without any bulky equipment and can realize numerous lab-on-a-chip applications with the integration of other microfluidic networks.